The embodiments described herein relate to thin-film device manufacturing, and more specifically to a method and resulting structure for providing thin-film devices on flexible substrates.
Modern semiconductor manufacturing techniques typically operate on a layer-by-layer basis, depositing and patterning one layer then the next layer above, and so on. The layers may be deposited, patterned, etched, etc. in order to produced an operational electronic structure such as an integrated circuit, sensor or display device, light emitter (light emitting diode—LED, solid state laser, etc.) and the like. Due to their composition of numerous thin layers, devices of this type which include at least one deposited layer are often referred to as thin-film structures, and the associated processes for producing them are often referred to as thin-film processes.
A typical layer of a thin-film structure is deposited and/or processed at a temperature in excess of so-called room temperature. Upon cooling of this layer following formation/processing intrinsic stresses often develop in the layer. One reason that it is common to form thin-film devices on a rigid substrate, such as glass, is the dimensional stability the rigid substrate offers to counter the intrinsic stress. The rigid substrate prevents the deposited film from curling and possibly cracking due to its intrinsic stress.
However, there has been a recent desire in the art for devices formed on flexible substrates such as plastic. Applications of such devices include conformal sensors, flexible (paperlike) displays, portable electronic devices, and so forth. The advantages of a flexible substrate over a rigid substrate are typically lighter weight, increased durability, and of course flexibility. However, manufacturing and processing thin-film devices on flexible substrates requires that careful attention be paid to the intrinsic stresses developed in the thin-film layers. As compared to rigid substrates, flexible substrates typically offer a lesser degree dimensional stability as a platform on which stressed layers may be formed. For example, the intrinsic stresses of thin-films formed on a flexible substrate can result in undesirable curling of the substrate and/or cracking or delamination of the thin-film layer(s).
For certain structures, such as the plastic often used for flexible substrates in thin-film device manufacturing (e.g., Teonex Q65A, a polyethylene napthalate film available from DuPont Teijin, www.dupontteijinfilms.com), heating relaxes internal stresses in the structure. This causes the structure to change dimension, for example expanding in length and/or width. This heat-induced movement is referred to as thermal expansion, and the coefficient of thermal expansion is defined as the degree of expansion divided by the change in temperature. When the structure cools, the stress rises in the structure, for example causing a shrinking in the structure's physical dimensions. In those temperature ranges in which the heating/cooling cycle results in instantaneous reversible dimensional change, i.e., where the structure returns to its original dimension upon cooling, the change in dimension is referred to as elastic deformation. If a critical temperature is exceeded, which depends on the material forming the structure, the cooled structure does not return to its original dimensions, but rather most often has increased final dimensions. This non-reversible deformation is referred to as plastic deformation. For many flexible substrate/thin-film processing systems, the processing temperatures exceed the elastic deformation temperature limit, resulting is a plastically deformed substrate as the processed structure cools. This makes alignment of the various layers very difficult, as the substrate on which the layers are formed changes dimensions from start of processing to end of processing.
More specifically, the layers and the substrate typically present different coefficients of thermal expansion. Thus, when heated, the layers and the substrate expand to differing degrees, or said another way, when the layers and the substrate cool, they change size by different degrees based on their different coefficients of the thermal expansion. This relative difference in degree of change when cooling often leads to a layer-to-layer mis-registration upon completion of the fabrication process, making multi-level photolithography difficult to impossible. Designing measures into the process to compensate for the different degrees of thermal expansion/contraction between substrate and deposited layers has proven to be a significantly difficult problem.
Thin-film thermo-mechanical analysis shows that the most direct method for minimizing layer-to-layer distortion during device fabrication is by adjusting the intrinsic, or built-in, stress in each deposited layer. This technique requires having precise control of the film deposition parameters and tailoring of the built-in stress for dimensional stability, which can result in narrowing of the design parameter range for fabrication of devices such as thin-film transistors (TFT). For example, alternating compensating layers that offset a compressive film with a tensile overlayer and vice-versa results in overall stress reduction in the device heterostructure. Lowering the initial stress-induced curvature, for example by fabricating at lower temperatures, reduces the need for subsequent deposition of strained multi-layers that are difficult to pattern. However, the lower temperatures limit the types of materials and process that can be used in device fabrication. Graded buffer layers have also been used for relieving built-in stress induced by thermal-coefficient mismatch between a substrate and the thin film. This technique is useful for single crystal material systems to control strain relaxation of multi-layer heterostructures. However, incorporation of this technique on elemental amorphous materials increases the complexity and cost of device manufacturing.
There is therefore a need in the art for a method and structure for providing improved compensation for thermally-induced expansion and contraction during thin-film device fabrication processes. More specifically, there is a need in the art for a method and structure for minimizing the relative changes in physical dimensions of a flexible substrate and typical thin-film layers formed thereover during the formation process such that during fabrication and upon completion of the fabrication process the various layers of a thin-film device are aligned as required for an operable device, without undue curling of the substrate, cracking of the deposited films, etc.